The present invention is an improvement in four-wheel-drive powertrains having an internal combustion engine and a multiple-ratio, geared transmission. The crankshaft of the engine is connected to torque input gearing of the transmission and the output shaft of the transmission is connected to a rear drive shaft, which delivers torque to a rear differential and to rear traction wheels for the vehicle.
A transfer case normally is used to provide a torque flow path to the front vehicle wheels of a four wheel drive vehicle or an all-wheel drive vehicle. The transfer case in a typical arrangement is assembled on the housing of the multiple-ratio transmission. A drive chain may be used to connect drivably a torque output element of the transmission to a torque input element of the transfer gear case. The power input shaft of the transfer gear case, in a typical environment, is connected to a sun gear of a planetary gear unit. The ring gear of the planetary gear unit is fixed. A carrier for such planetary gear units carries a pinion assembly comprising pinions drivably engaged with the sun gear and the ring gear. The carrier is the torque output element of the transfer case planetary gearing.
A positive engagement clutch may be used to connect drivably the rear output shaft of the transfer case to the power input shaft for the transfer case, the latter being connected to the output of the multiple-ratio transmission. When the positive engagement clutch is shifted in the opposite direction, the power input sun gear drives the rear output shaft for the transfer case through the planetary gearing as the carrier becomes connected to the rear output shaft. When the positive engagement clutch is moved to an intermediate position, the torque delivery path from the power input shaft for the transfer case is interrupted and a connection between the rear output shaft and the carrier is disestablished, placing the drivetrain in a "neutral" state.
The transfer case has three operating modes; i.e., a two-wheel mode, a four-wheel high-speed mode and a four-wheel low-speed mode. A neutral position exists between the four wheel high and four wheel low modes. A secondary clutch assembly is used to disconnect the drive chain from the rear output shaft if it is desired to operate the vehicle in the two-wheel-drive mode. A motor is provided to shift the transfer case between operating modes.
The transfer case may not complete a shift to the desired mode due to several factors, including the motor stopping before the shift is complete, the driver moving a switch to change the desired operating mode before a shift is complete, and an error which instructs the motor not to complete a shift.
U.S. Pat. No. 4,664,217 to Welch et al describes a system where a controller requires the vehicle speed to be approximately zero and the transmission to be in a neutral state for a time of 0.1-0.5 seconds when shifting the transfer case between neutral and either 4H or 4L, or vice-versa, then the transfer case may be shifted into a selected 4 wheel drive mode. However, the '217 patent does not provide further safeguards included in the present disclosure to protect the transfer case and to prevent the transfer case from being left in a neutral state.
It would be desirable to provide further protection for the transfer case and to prevent the transfer case from being left in a neutral state and to provide an operating strategy in the event of error detection.